Thursday, 28 October 2010

Supporting alternative formatting styles in logging

When logging was added to the Python standard library, the only way of formatting messages with variable content was to use the %-formatting method. Since then, Python has gained two new formatting approaches: string.Template (added in Python 2.4) and str.format (added in Python 2.6).

Now, Python’s logging package will provide improved support for these two additional formatting styles. The Formatter class in logging has been enhanced for Python 3.2 to take an additional, optional keyword parameter named style. This defaults to '%', but other possible values are '{' and '$', which correspond to the other two formatting styles. Backwards compatibility is maintained by default (as you would expect), but by explicitly specifying a style parameter , you get the ability to specify format strings which work with str.format or string.Template. Here’s an example console session to show the possibilities:

vinay@eta-jaunty:~$ python3.2
Python 3.2a3+ (py3k:85857, Oct 27 2010, 18:25:50)
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.

>>> import logging
>>> root = logging.getLogger()
>>> root.setLevel(logging.DEBUG)
>>> handler = logging.StreamHandler()
>>> bf = logging.Formatter('{asctime} {name} {levelname:8s} {message}', style='{')
>>> handler.setFormatter(bf)
>>> root.addHandler(handler)
>>> logger = logging.getLogger('')
>>> logger.debug('This is a DEBUG message')
2010-10-28 15:11:55,341 DEBUG This is a DEBUG message
>>> logger.critical('This is a CRITICAL message')
2010-10-28 15:12:11,526 CRITICAL This is a CRITICAL message
>>> df = logging.Formatter('$asctime $name ${levelname} $message', style='$')
>>> handler.setFormatter(df)
>>> logger.debug('This is a DEBUG message')
2010-10-28 15:13:06,924 DEBUG This is a DEBUG message
>>> logger.critical('This is a CRITICAL message')
2010-10-28 15:13:11,494 CRITICAL This is a CRITICAL message

Note that the formatting of logging messages for final output to logs is completely independent of how an individual logging message is constructed. That can still use %-formatting, as shown here:

>>> logger.error('This is an%s %s %s', 'other,', 'ERROR,', 'message')
2010-10-28 15:19:29,833 ERROR This is another, ERROR, message

Logging calls (logger.debug(), etc.) only take positional parameters for the actual logging message itself, with keyword parameters used only for determining options for how to handle the actual logging call (e.g. the exc_info keyword parameter to indicate that traceback information should be logged, or the extra keyword parameter to indicate additional contextual information to be added to the log). So you cannot directly make logging calls using str.format or string.Template syntax, because internally the logging package uses %-formatting to merge the format string and the variable arguments. There would no changing this while preserving backward compatibility, since all logging calls which are out there in existing code will be using %-format strings.

There is, however, a way that you can use {}- and $- formatting to construct your individual log messages. Recall that for a message you can use an arbitrary object as a message format string, and that the logging package will call str() on that object to get the actual format string. Consider the following two classes:

class BraceMessage(object):
def __init__(self, fmt, *args, **kwargs):
self.fmt = fmt
self.args = args
self.kwargs = kwargs

def __str__(self):
return self.fmt.format(*self.args, **self.kwargs)

class DollarMessage(object):
def __init__(self, fmt, **kwargs):
self.fmt = fmt
self.kwargs = kwargs

def __str__(self):
from string import Template
return Template(self.fmt).substitute(**self.kwargs)

Either of these can be used in place of a format string, to allow {}- or $-formatting to be used to build the actual “message” part which appears in the formatted log output in place of “%(message)s” or “{message}” or “$message”. It’s a little unwieldy to use the class names whenever you want to log something, but it’s quite palatable if you use an alias such as __ (double underscore – not to be confused with _, the single underscore used as a synonym/alias for gettext.gettext or its brethren).

The above classes, and recent additions to the logging package, will be published in a package called logutils, intended for use with Python 2.4 or later. This is already available for download via PyPI (and will be the subject of a future blog post). They can be used as follows:

>>> from logutils import BraceMessage as __

>>> print(__('Message with {0} {1}', 2, 'placeholders'))
Message with 2 placeholders
>>> class Point: pass
>>> p = Point()
>>> p.x = 0.5
>>> p.y = 0.5
>>> print(__('Message with coordinates: ({point.x:.2f}, {point.y:.2f})', point=p))
Message with coordinates: (0.50, 0.50)
>>> from logutils import DollarMessage as __
>>> print(__('Message with $num $what', num=2, what='placeholders'))
Message with 2 placeholders

One thing to note is that you pay no significant performance penalty with this approach: the actual formatting happens not when you make the logging call, but when (and if) the logged message is actually about to be output to a log by a handler. So the only slightly unusual thing which might trip you up is that the parentheses go around the format string and the arguments, not just the format string. That’s because the __ notation is just syntax sugar for a constructor call to one of the XXXMessage classes.

Your comments are, as always, welcome. Thanks for reading.


  1. Why can't you allow new style formatting for logging calls, restricting them to positional arguments? That's the same restriction that's already in place for %-style formatting, isn't it?

    The style for message creation could be specific to each Logger, which would hand it down to LogRecord objects, so that their getMessage knows what style to use.

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